International patent application PCT/ SE88/00653 discloses a method for developing pictures and text with pigment particles on an information carrier, directly from computer generated signals, without the need for these signals to be intermediately stored for temporary conversion to light energy, which is the case in photo conductive printers, e.g. laser printers. These problems have been solved by bringing the information carrier into electrical cooperation with at least a screen or lattice-shaped, preferably an electrode matrix, which through control, in accordance with the configuration of the desired pattern, at least partly opens and closes passages through the matrix which is galvanically connected to a voltage source. An electric field is exposed through the open passages for attraction of the pigment particles against the information carrier.
This method (in the following called the EMS- concept), as it is described in the above patent application, however may result in produced print which does not have high quality, particularly with repeated and continuous use.
The EMS-concept refers to electrode matrices in which passages or meshes through the matrix are defined and separated by simple electrodes, which results in that the potential of every single electrode substantially influences the characteristics of the electric field on the pigment particles symmetrically in passages adjoining the electrodes. This results in the attraction of pigment particles (in the following called toner), not only in the mesh, which is surrounded by electrodes, the potential of which is intended to completely or partly open said mesh (in the following called "black" voltage), but also to expose passages in adjacent meshes. In electrode matrices with several mesh lines, meshes surrounded by simple electrodes will develop full-dots with intended extension and position, as well as half-dots and quarter-dots surrounding the full-dots. This results in an unsatisfactory edge definition and in certain cases as a "blur" on the printed page. It is possible to change the potential of the adjacent electrodes, which are intended to close the passage in the adjacent meshes (in the following called "white" voltage) and hereby reduce the problem with the undesired half- and quarter-dots, by skew setting the above mentioned symmetrical influence on the electrical field. This however leads to a potential difference between electrodes with "white" voltage and electrodes with "black" voltage increases (in the following called contrast voltage), which in turn increases the manufacturing costs for the control electronics, as well as the electrode matrix.
These problems stated above are not limited to the EMS- concept but are also present wholly or partially in several electrographic printer concepts, where passages of toner is created in an electrical manner.
Common to all problems described here, another drawback of the known technique is that the printing quality, and thereby the readability, is influenced in a negative direction resulting in reduced competitiveness and lower consumer value.